Multiple lines of evidence suggest that down-modulation of the viral receptor in HIV- infected cells is an essential function modulating pathogenesis in vivo. We have previously performed proof-of-concept studies demonstrating that interference with this function results in the production of non-infectious particles unable to replicate in permissive cells. Here we propose to screen a library of small molecules to identify inhibitors of CD4 down-modulation. We will use a simple screening assay that recapitulates physiological conditions and use natural target cells of infection. We propose the following aims: (1) to optimize an assay for CD4 down-modulation in HIV-infected PBMC;(2) to screen a library of 20,000 drug-like compounds, and;(3) to validate and characterize the potency, cytotoxicity and specificity of hits. Studies will be implemented to optimize screening conditions, reduce assay-to-assay variability and perform statistical analysis to set thresholds for hit identification. False positives and artifacts will be discriminated in secondary assays, and true hit compounds will be prioritized based on potency, specificity, and lack of cytotoxicity. Preliminary studies will be performed to validate the mechanism of action and test the compounds antiviral activity with laboratory adapted HIV strains and primary isolates. Success in this Phase I application is defined as the milestone identification of one or more bona fide lead series of drug-like compounds with IC50s below 5 5M in HIV replication assays and minimal cytotoxicity (CC50 10-fold greater than IC50). Lead compounds with these profiles will be optimized for potency, specificity and bioavailability in a Phase II project aimed at advancing the preclinical development of these compounds to support human trials. These studies will constitute the foundation of a drug discovery program that may result in the characterization of first-in-class drugs of a novel family of antiretrovirals. PUBLIC HEALTH RELEVANCE: As of the end of 2007, 33 million people worldwide are infected with HIV. Despite advances in antiretroviral therapies, side-effects and the emergence of viruses resistant to currently available drugs pose important problems for the treatment of HIV. We propose to identify and characterize antiretroviral compounds that block HIV through a novel mechanism. These unique compounds could be used for the treatment of HIV infection.